Thailand's anti-government protesters and security forces have observed a temporary truce as the nation prepares to mark the birthday of the revered king, after police stepped back in a dramatic move to calm violent clashes.

Demonstrators joined Bangkok authorities to clean up the area around Democracy Monument on Wednesday, where tens of thousands have camped out in more than a month of rallies against the embattled government of Prime Minister Yingluck Shinawatra.

The area is a focus for Thursday's celebrations that will mark the 86th birthday of King Bhumibol Adulyadej, a date normally observed in a spirit of calm and reverence for the ageing monarch.

Some debris has been cleared at battle-scarred areas around key government buildings, which saw ugly clashes with police earlier in the week.

Several days of street battles in the Thai capital abruptly paused on Tuesday, as protesters gave police flowers after security forces said they would no longer use force against demonstrators trying to storm Yingluck's offices as well as the city police headquarters.

The unexpected about-turn in tactics followed two days of violent clashes between stone-throwing mobs and police firing tear gas, water cannons and rubber bullets.

Protest leaders say they have not abandoned their campaign to overthrow Yingluck's administration and curb the political influence of her billionaire brother Thaksin Shinawatra, who was deposed by royalist generals in a coup seven years ago.

Despite the apparent respite, groups of marchers began to arrive at the headquarters of the national police in Bangkok's city centre shopping district on Wednesday, although the rally there was expected to be brief and peaceful.

Authorities were not worried about the gathering, said Paradorn Pattanatabut, the head of Thailand's National Security Council.

"Their movement today is merely to display their power and show that the protests are still continuing," he added.